5. Explain the process of image acquisition.

Tuesday, 5 March 2013

The types of images in which we are interested are generated by the combination of an
“illumination” source and the reflection or absorption of energy from that source by the elements
of the “scene” being imaged. We enclose illumination and scene in quotes to emphasize the fact
that they are considerably more general than the familiar situation in which a visible light source
illuminates a common everyday 3-D (three-dimensional) scene. For example, the illumination
may originate from a source of electromagnetic energy such as radar, infrared, or X-ray energy.
But, as noted earlier, it could originate from less traditional sources, such as ultrasound or even a
computer-generated illumination pattern.

Similarly, the scene elements could be familiar objects, but they can just as easily be molecules,
buried rock formations, or a human brain. We could even image a source, such as acquiring
images of the sun. Depending on the nature of the source, illumination energy is reflected from,
or transmitted through, objects. An example in the first category is light reflected from a planar
surface. An example in the second category is when X-rays pass through a patient’s body for the
purpose of generating a diagnostic X-ray film. In some applications, the reflected or transmitted
energy is focused onto a photo converter (e.g., a phosphor screen), which converts the energy
into visible light. Electron microscopy and some applications of gamma imaging use this
approach.

Figure 5.1 shows the three principal sensor arrangements used to transform illumination energy
into digital images. The idea is simple: Incoming energy is transformed into a voltage by the
combination of input electrical power and sensor material that is responsive to the particular type
of energy being detected. The output voltage waveform is the response of the sensor(s), and a
digital quantity is obtained from each sensor by digitizing its response.

Fig.5.1 (a) Single imaging Sensor (b) Line sensor (c) Array sensor

(1)Image Acquisition Using a Single Sensor:

Figure 5.1 (a) shows the components of a single sensor. Perhaps the most familiar sensor of this

type is the photodiode, which is constructed of silicon materials and whose output voltage
waveform is proportional to light. The use of a filter in front of a sensor improves selectivity. For
example, a green (pass) filter in front of a light sensor favors light in the green band of the color spectrum. As a consequence, the sensor output will be stronger for green light than for other
components in the visible spectrum

In order to generate a 2-D image using a single sensor, there has to be relative displacements in

both the x- and y-directions between the sensor and the area to be imaged. Figure 5.2 shows an
arrangement used in high-precision scanning, where a film negative is mounted onto a drum
whose mechanical rotation provides displacement in one dimension. The single sensor is
mounted on a lead screw that provides motion in the perpendicular direction. Since mechanical
motion can be controlled with high precision, this method is an inexpensive (but slow) way to
obtain high-resolution images. Other similar mechanical arrangements use a flat bed, with the
sensor moving in two linear directions. These types of mechanical digitizers sometimes are
referred to as microdensitometers.

Fig.5.2. Combining a single sensor with motion to generate a 2-D image

(2) Image Acquisition Using Sensor Strips:

A geometry that is used much more frequently than single sensors consists of an in-line

arrangement of sensors in the form of a sensor strip, as Fig. 5.1 (b) shows. The strip provides
imaging elements in one direction. Motion perpendicular to the strip provides imaging in the
other direction, as shown in Fig. 5.3 (a).This is the type of arrangement used in most flat bed
scanners. Sensing devices with 4000 or more in-line sensors are possible. In-line sensors are used routinely in airborne imaging applications, in which the imaging system is mounted on an
aircraft that flies at a constant altitude and speed over the geographical area to be imaged. Onedimensional imaging sensor strips that respond to various bands of the electromagnetic spectrum are mounted perpendicular to the direction of flight. The imaging strip gives one line of an image

at a time, and the motion of the strip completes the other dimension of a two-dimensional image.

Lenses or other focusing schemes are used to project the area to be scanned onto the sensors.

Sensor strips mounted in a ring configuration are used in medical and industrial imaging to

obtain cross-sectional (“slice”) images of 3-D objects, as Fig. 5.3 (b) shows. A rotating X-ray
source provides illumination and the portion of the sensors opposite the source collect the X-ray
energy that pass through the object (the sensors obviously have to be sensitive to X-ray
energy).This is the basis for medical and industrial computerized axial tomography (CAT). It is
important to note that the output of the sensors must be processed by reconstruction algorithms
whose objective is to transform the sensed data into meaningful cross-sectional images.

In other words, images are not obtained directly from the sensors by motion alone; they require

extensive processing. A 3-D digital volume consisting of stacked images is generated as the
object is moved in a direction perpendicular to the sensor ring. Other modalities of imaging
based on the CAT principle include magnetic resonance imaging (MRI) and positron emission
tomography (PET).The illumination sources, sensors, and types of images are different, but
conceptually they are very similar to the basic imaging approach shown in Fig. 5.3 (b).

Figure 5.1 (c) shows individual sensors arranged in the form of a 2-D array. Numerous
electromagnetic and some ultrasonic sensing devices frequently are arranged in an array format.
This is also the predominant arrangement found in digital cameras. A typical sensor for these
cameras is a CCD array, which can be manufactured with a broad range of sensing properties
and can be packaged in rugged arrays of 4000 * 4000 elements or more. CCD sensors are used
widely in digital cameras and other light sensing instruments. The response of each sensor is
proportional to the integral of the light energy projected onto the surface of the sensor, a property
that is used in astronomical and other applications requiring low noise images. Noise reduction is
achieved by letting the sensor integrate the input light signal over minutes or even hours. Since
the sensor array shown in Fig. 5.4 (c) is two dimensional, its key advantage is that a complete
image can be obtained by focusing the energy pattern onto the surface of the array. The principal
manner in which array sensors are used is shown in Fig.5.4. This figure shows the energy from
an illumination source being reflected from a scene element, but, as mentioned at the beginning
of this section, the energy also could be transmitted through the scene elements. The first
function performed by the imaging system shown in Fig.5.4 (c) is to collect the incoming energy
and focus it onto an image plane. If the illumination is light, the front end of the imaging system
is a lens, which projects the viewed scene onto the lens focal plane, as Fig. 2.15(d) shows. The
sensor array, which is coincident with the focal plane, produces outputs proportional to the
integral of the light received at each sensor. Digital and analog circuitry sweep these outputs and
converts them to a video signal, which is then digitized by another section of the imaging system.
The output is a digital image, as shown diagrammatically in Fig. 5.4 (e).